1988 Toyota Pickup

1988 toyota pickup, 239,000 miles; I bought 5 years ago. I have put less then 10,000 miles on it in 5 years. Previous owner said he overhauled it before selling to me. I have had no problems until this week. When I am on any slight incline the speed starts dropping and I have to downshift. Today I was on hwy and going 65 mph and started going uphill and dropped down to 30 (while downshifting) until hwy leveled out and started gaining speed again. 4 cylindars. 5 speed manual. No smell of burning clutch.
Should I give a tune up first?
Should I replace fuel filter first? Oxygen sensor?
What should I do and in what order?
thanks,
scott b.